The debt ceiling is the highest amount of money the U.S. government is legally allowed to borrow to pay back its creditors for bills. Uncle Sam borrows money to pay interest on the national debt, Social Security and Medicare benefits, and a lot more. Failure to reach a debt-ceiling agreement in 2011 resulted in S&P downgrading the U.S.'s credit rating — causing "a clear and deep dent in U.S. economic and market data," said Matt Phillips at Quartz.

Economists say this time could be even worse. A sampling of terrifying prophecies:

Prepare for a "financial apocalypse," says Yalman Onaran at Bloomberg.

Failure by the world's largest borrower to pay its debt — unprecedented in modern history — will devastate stock markets from Brazil to Zurich, halt a $5 trillion lending mechanism for investors who rely on Treasuries, blow up borrowing costs for billions of people and companies, ravage the dollar and throw the U.S. and world economies into a recession that probably would become a depression. Among the dozens of money managers, economists, bankers, traders, and former government officials interviewed for this story, few view a U.S. default as anything but a financial apocalypse. [Bloomberg]

You should find the prospect of a debt default "terrifying," says Matt Yglesias at Slate.

Nobody really knows what will happen if we breach the debt ceiling because it's never happened before. And everyone worries that it will be awful because nobody's created any legal provision for not making it awful...

A debt ceiling breach... is completely unprecedented. There's no guarantee that it'll lead to a worldwide financial panic and a massive global depression, but there's honestly no guarantee that it won't. Nobody knows what will happen, and you should find that prospect terrifying. [Slate]

Mark my words, says economist Justin Wolfers: This would plunge the U.S. into a recession.

If you hit the debt ceiling, then you can't borrow any more money, which means you've got to bring the budget deficit down to zero immediately. It's currently four percent of GDP. So this would cause a huge fiscal contraction if the debt ceiling limit wasn't raised for a persistent period. Big enough that I can be absolutely confident it would plunge the U.S. into recession. [ABC]

If U.S. credit faltered, lenders around the world would think twice about making loans, including the ones companies rely on to make payroll every week.

"This would be a financial collapse and chaos unlike anything anybody has seen before," Hoffman said. "It's unthinkable." [AZ Central]

Here's the scariest thing, says Derek Thompson at The Atlantic: No one knows how bad it will be.

The truly scary thing about going over the debt cliff isn't what we think will happen — a scramble to prioritize payments, delayed checks to groups like veterans and senior citizens, and angry, confused investors.

The truly scary thing is that we actually have no idea what will happen. We don't know if it's even possible for the government to prioritize payments to millions of different clients. Households, businesses, and investors don't know how long they'll have to wait for their money, whether it's a defense contract deal, a doctor's reimbursement, or a Social Security check. And nobody will know how long the nightmare will go on. [The Atlantic]

And a counterpoint (sort of), from Warren Buffet.

Last week, the legendary investor told Squawk Box that if Democrats and Republicans fail to reach an agreement in time, that would be "pretty damn dumb." But he doesn't see that happening. "The market is not gonna fall apart, because [investors] expect Washington will only act irrationally for a certain length of time," he said.